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From: Mark Lloyd
Subject: Re: Triac controlled holiday lights
Date: 3 Dec 2002 17:27:07 -0600
References: <firstname.lastname@example.org> <email@example.com> <3DEB8034.firstname.lastname@example.org> <email@example.com> <3DECCCDF.firstname.lastname@example.org>
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On Tue, 03 Dec 2002 15:24:31 GMT, Fred Bloggs
>Steve W.EE wrote:
>> In comp.home.automation Fred Bloggs wrote:
>> : Steve W.EE wrote:
>> : [...snip extraneous stuff...]
>> : Given your capability, the surest route to a finished project is:
>> : 1) power up the string of 50 bulbs at 120VAC and measure the AC current Iac;
>> : 2) compute resistor R= 60VAC/Iac ohms
>> : 3) compute resistor power rating P=R x (Iac)^2
>> : 4) select standard resistor value Rs closest to computed R and standard
>> : power rating no less than 1.5 x P
>> : 5) insert resistor Rs in series with string of 25 bulbs and power of 120VAC.
>> : example: string of 50 bulbs is 50W then you have Iac will be measured at
>> : 50W/120Vac=0.42 Amps. This makes R=60/0.42= 144 ohms, and power= 25
>> : Watts. Place a 150 ohm 50W power resistor in series with each string of
>> : 25 bulbs, or use 2x 75 ohm 20W in series with string etc...
>> I did the calculation, what I need is a 1100 ohm, 8 watt resistor.
>> Actually, 14 of them. These are not exactly off-the-shelf items. It
>> has to be something which is properly heat-sinked, and suitable to
>> use outdoors. Any ideas on where to get them?
>Is 8 Watt the actual power dissipation? If so then go with 1.1K at 20W,
>these are available for about 30 cents, reasonable size, and called
>"cement" power resistors in the trade. No heat sinks will be required.
>Look in your yellow pages under electronic equip. & supls.- dlrs to find
>a retailer who sells to electronics repair shops. They will have the NTE
>line in cement power. You can place them on a small perf board that
>mounts under/over your triac control board on standoffs.
>> Actually, I did find a place where I could get them for about 30
>> cents apiece, including water-resistant heat sinks, a mounting
>> bracket and even some quality connecting wire already attached.
>> There is some type of unwanted radiation they might give off but
>> it will not be difficult to fabricate a shield.
>> Care to take a guess where I plan to get these resistors?
>> : Your idea with the diode(s) will pop the bulbs in about 10 minutes.
>> I beg to disagree on this. It does not harm incandescent bulbs to
>> drive them with half-wave AC. In fact it lengthens their life because
>> they are burning at a lower level. In fact a few years back
>> they were selling diodes in a small flat package which could be placed
>> in a lightbulb socket to dim the lights and save energy.
>Right- but those were for bulbs rated for 120VAC. The half-wave
>rectification cuts the RMS down to 70% of that or 84VAC.
I suspect you may be confusing different ways of measuring voltage.
The rectification is cutting out HALF (50%) of the power. This means
that the filaments will be heated half as much.
> In your case,
>you would have 84VAC across a string rated for 60VAC, and this will be
>bad news. My dimensions were off in the original post, but if the bulbs
>are rated at 700 hours on 60VAC, then 84VAC will reduce their lifetime
>to 12 hours.
>> The reason I'm going to abandon the plan with the diodes, it's not
>> practical. If I took a string of bulbs it would have to be cut
>> to 71% of its length. Then when driven through a diode the bulbs
>> would be at full brightness. A 50 bulb string would be cut to 35
>> bulbs, this is not enough a reduction to we worthwhile.
>Ahh- that's an idea.
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